Generally, surface properties can be classified into hydrophobic and hydrophilic surfaces depending on the value of the water contact angle (WCA). A surface having a WCA greater than 90° is referred as hydrophobic, whereas a WCA smaller than 90° is referred as hydrophilic. Maximum water contact angle on a smooth surface is about 120°. In practice, two types of WCA values are used: static and dynamic. Static water contact angles (θst) are obtained by sessile drop measurements, where a drop is deposited on the surface and the value is obtained by a goniometer or a specialized software. Dynamic contact angles are non-equilibrium contact angles and are measured during the growth (advancing WCA θadv) and shrinkage (receding WCA θrec) of a water droplet. The difference between θadv and θrec is defined as contact angle hysteresis (CAH).
Superhydrophobic surfaces are characterized by extreme water repellency with WCAs greater than 150°, and a CAH of less than 10°. On the contrary, water spreads immediately on superhydrophilic surfaces leading to WCAs less than 10°. Both superhydrophobicity and superhydrophilicity are the result of a combination of high surface roughness with either hydrophobic or hydrophilic material, respectively. Combining these two extreme properties on the same surface in precise two-dimensional micropatterns opens exciting new functionalities and possibilities in a wide variety of applications from cell,[1-3] droplet,[2,4] and hydrogel microarrays[2,5] for screening to surface tension-confined microfluidic channels for separation and diagnostic devices.[6]
Further, continuous superhydrophobic coatings are of high importance for different industrial applications ranging from self-cleaning, anti-icing or anti-corrosion coatings to membranes for oil-water separations or clothing coatings. Most of such superhydrophobic coatings have to be applied to large areas and do not need to be further functionalized. However, for example biotechnological and biomedical applications commonly require superhydrophobic coatings that can be further modified to create nano- to micro-patterns of either different chemical functionalities or physical properties, such as the aforementioned superhydrophobic-superhydrophilic micropatterns. For example, in US 2007/0135007 A1 and WO 2004/113456 A2, methods for producing superhydrophobic coatings are described employing alkyl silanes to fabricate the respective coatings. However, if alkyl silanes are present on the surface, post-functionalization on the coated surface is not easy to be achieved as the alkyl silane coated surface is stable, thereby needing very harsh conditions to post-modify the surface.
A number of methods for making superhydrophobic-superhydrophilic micropatterns have been described over the past decade. For example, methods based on UV-induced decomposition of hydrophobic coatings on different substrates, such as alumina, TiO2 film or SiO2, were reported.[7] Photoinduced modification of carbon nanotubes with hydrophilic azides,[8] plasma treatment,[9] microprinting,[10] or mussel-mimetic deposition of dopamine in combination with soft-lithography[11] have been described. Recently, a method based on UV-initiated photografting for making superhydrophobic-superhydrophilic micropatterns on porous polymer films has been described[1,12], as it is also disclosed in EP 2 481 794 A1. Further, an amine reactive superhydrophobic surface that permits post-fabrication by amine-functionalized molecules has been reported.[13]
Most of the described methods, however, proceed slowly as, for instance, an irradiation time of 30 min is required in the case of the UV-initiated photografting approach described in EP 2 481 794 A1. Moreover, the above-mentioned methods lack the ability to easily tailor or modify the properties by different target functional groups, or require harsh conditions (e.g. plasma treatment or UV-induced decomposition), organic solvents (i.e. incompatible with aqueous conditions) and, therefore, cannot be directly applied to make patterns of biomolecules. These limitations restrict the range and number of possible practical applications of produced superhydrophobic-superhydrophilic micropatterns.
Moreover, methods based on physisorption of hydrophilic chemicals on superhydrophobic surfaces to create hydrophilic patterns usually suffer from short term stability of such coatings. Another approach to achieve controllable and facile functionalization of a superhydrophobic surface is to introduce reactive functional groups into the superhydrophobic coating. However, the presence of reactive functional groups often leads to the increase in surface energy, resulting in the loss of superhydrophobicity.
Principally, besides meeting several requirements, such as having sufficient transparency and a flat topography, another important criterion for microarrays and cell arrays, respectively, is that the spot density should be sufficiently high. However, by increasing the spot density so as to provide, for instance, a genome-on-a-chip cell array, several challenges still exist. Firstly, with increasing spot density and thus, decreasing the distance between spots, cross contamination becomes a serious problem. Secondly, high density packing of the spots requires non-circular (e.g. square) geometry of the spots, which sets additional constraints on the ability to control surface chemistry. The spot size depends, inter alia, on the surface tension of spotting solutions that may differ from one probe-molecule-cocktail to another. This may lead to the variation of the spot size and therefore limits the minimum distance between spots. Another serious problem, arising from the high proximity of spots in a high-density cell array, is the migration of transfected cells between adjacent spots that causes growing systematical errors with smaller spot-to-spot distance. Developing a universal method that is facile, versatile, as well as provides good optical and chemical surface properties remains a big challenge.